Notes on origins and biogeomorphology of Montgomery Reef, Kimberley, Western Australia

Journal of the Royal Society of Western Australia, 94:107-119, 2011

Notes on the origins and biogeomorphology of Montgomery Reef,

Kimberley, Western Australia

B Wilson 1 &S Blake 2

Research Associate, Western Australian Museum, Perth
2 Western Australian Marine Science Institution, Perth
Corresponding author: EE3 murexpecten@bigpond.com

Manuscript received December 2010; accepted April 2011

Abstract

Montgomery Reef, lying at the boundary of Camden Sound and Collier Bay is a very large rock
platform (c. 400 km 2 ) in an open sea setting of the Kimberley Bioregion, Western Australia. It is not
a coral reef platform in the strict sense but an ancient terrestrial structure, probably a flat-topped
mesa, with a Holocene veneer of marine biogenic sediments superimposed over inherited terrestrial
geomorphic features. The eastern end of the reef, at least, has base rocks (beneath the coralgal
veneer) of dolomite, underlying quartz sandstone mapped as Pentecost Sandstone, an upper
member of the Paleoproterozoic Kimberley Group. The dolomite is an unrecognised formation and
its well preserved stromatolites are undescribed. Although the coral fauna on the reef platform is
moderately diverse, coral reef-building is located primarily in the impounded pools lagoons of the
reef platform. There is very little coral growth on the reef-front. An unusual feature is the relative
importance of rhodoliths that form massive containment banks around the perimeter of the reef
and are responsible for creating the high lagoon habitats. Field observations suggest that rhodoliths
may be the most important contemporary reef-builders on Montgomery Reef with very high
primary production inferred.

Keywords: Montgomery Reef, Holocene reef growth, rhodoliths, corals, stromatolites,
biogeomorphology

Introduction

Montgomery Reef is a very large, flat-topped
geomorphic structure whose platform surface is exposed
at low tide. It has an estimated area of c. 400 km 2 , located
in open sea at the Collier Bay-Camden Sound boundary
about 15 nautical miles north of the Yampi Peninsula.
Recent surveys of the modern reef fauna of the
Montgomery Reef platform have been carried out by the
Australian Institute of Marine Science and the Western
Australian Museum, rendering this one of the better
known reefs in the Kimberley in regard to its biota.
However, its geology and reef-building processes remain
undescribed. There have been various interpretations of
the nature of the reef and its origins. Teichert and
Fairbridge (1948) referred to it as one of several "large,
rather irregular patches of reefs [that] rise in the deeply
indented bays of the Kimberley coast". Burbidge el al.
(1991) noted that "the Montgomery Islands at the centre
of the platform are surrounded by "sand flats and coral
reef". The Marine Parks and Reserves Selection Working
Group referred to Montgomery Reef as "an extensive
intertidal and shallow subtidal rock platform" (CALM
1994). Brooke (1997) described it as one of several large
"carbonate bioherms" near the Kimberley coastline. This
report considers the nature of the reef and its origins and
reef-building processes on the basis of field observations
made during a brief visit.

© Royal Society of Western Australia 2011

The geological and biogeomorphic history of
Montgomery Reef and its biota have particular
importance because its small islands have high cultural
significance. O'Connor (1994) excavated material from an
open habitation site on the largest of the High Cliffy
Islands (Ngalanguru) and dated it at round 6,700 years
B.P., indicating that the islands of this reef were
populated by people with a specialised maritime
economy soon after the end of the post-glacial
transgression. Since that time, people lived there, at least
periodically, until the "contact" period in the early part
of the 20 lh century. It is said that these people, who were
known as the Jaudibaia, spoke a distinctive dialect (Love,
quoted by Tindale 1974) indicating a long independent
heritage. The resources that supported this group were
primarily those associated with the reef.

Methods

This report is largely descriptive, presenting field
observations from a brief visit to Montgomery Reef by a
party from the Western Australian Marine Science
Institution [WAMS1] in August-September, 2009 aboard
the W.A. Fisheries patrol vessel P.V. Walcott.
Hyperspectral airborne imaging of Montgomery Reef,
including tri-colour scanner and high resolution digital
photographs of the study area commissioned by WAMS1
were produced and processed by Airborne Research
Australia and Curtin University Department of Applied
Physics. These data provide the first high resolution

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Journal of the Royal Society of Western Australia, 94(2), June 2011

Figure 1. Montgomery Reef. Acknowledgements to Google Earth.

digital base map with 3 metre on ground resolution that
can be further interrogated for detailed spectral
information from the key "end members" including
corals, algae, rhodoliths, sediments and mangroves.

Terminology

In this report rock platform is used as a geomorphic
term meaning a reef structure that has a near-horizontal
surface in the intertidal zone, the rock being of any kind.
Such a platform surface may have been an outcome of
erosional or constructional processes, or a combination
thereof. Intertidal rock platforms are thought to be
erosional and are often referred to as "wave-cut
platforms" (Edwards 1958). A coral reef platform is one
where the framework of the structure is constructed, in
situ, by growth of coral and calcareous algae and the
level of the platform surface is determined by limits to
vertical coral growth. The term coral platform reef is a
category of coral reef that lacks a lagoon and commonly
has an oval outline, a central sand island and a flat
intertidal surface. On the Kimberley coast there are many
intertidal rock platforms of flat-bedded Proterozoic rocks
that have a wedge of Holocene coral and algal growth
and carbonate deposition at the reef-front, thereby

warranting the term fringing reef Rock and coral reef
platforms generally have a sloping lower-littoral reef-front
ramp with a distinct reef edge, a reef crest and a mid-littoral
reefflat .

As a geomorphic structure, Montgomery Reef and its
parts do not fit, exactly, any of these terms and it is
necessary to describe what is observed and use standard
reef terminology with caution and explanation.

Results

General morphology of Montgomery Reef

Montgomery Reef has a horizontal but terraced
surface within the intertidal zone, referred to here as the
reef platform, and steep, cliff-like peripheral walls in the
subtidal fore-reef zone. Three primary biogeomorphic
units may be distinguished, the main reef, a north¬
pointing arm called The Breakwater at its western end,
and a small satellite reef separated by a deep channel at
its eastern end called High Cliffy Reef (Fig. 1). There are
several small islands on the platform. The Montgomery
Islands comprise a cluster of low, mud islands at its
centre, vegetated with mangroves and grassy flats and

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Wilson & Blake: Biogeomorphology of Montgomery Reef, Kimberley

Figure 2. High Cliffy Reef with its five small islands, separated
from the main Montgomery Reef platform by a deep channel.
(Data courtesy of WAMSI and Airborne Research Australia, and
images processed by Curtin University.)

largely surrounded by sand sheets. There are also eight
small rocky islands on the eastern margin of the complex.
They include five islands, known collectively as the High
Cliffy Islands, arranged around the eastern and southern
fringe of High Cliffy Reef (Fig. 2). The other three are
located on or near the eastern margin of the main
Montgomery Reef and are known as the dual Egret
Islands (Fig. 3) (referred to as Jungadi by Roy Wiggan)
and Wulajarlu Island (Fig. 4).

The form of the main reef platform is atypical. There
is very little coral reef growth at the reef-front and
calcareous algal growth appears to dominate biogenic
carbonate deposition on the reef flat. There are some
parts of the reef flat where rock pavement (with a
crustose algal veneer) is exposed at low tide but most of
its vast area (c. 350 km 2 ) is occupied by a shallow lagoon
and pools. The reef flat and its lagoon and pools are at
two levels separated by an impoundment bank of
rhodoliths, the upper level about 50 cm above the lower
one behind the reef crest. There is no back-reef and in
that respect Montgomery Reef resembles a very large
coral platform reef. Some details are given in the
following section on intertidal habitats.

Geology

The mainland coast and islands east and south of
Montgomery Reef lie along the south-western margin of
the Kimberley Basin, its rocks being upper members of

the Paleoproterozoic Kimberley Group, namely, the
Pentecost Sandstone, Elgee Siltstone and Yampi
Formation and intrusive igneous rocks. By its location it
might be expected that the base rocks of the Montgomery
Reef complex would belong in this series.

The Islands

The low Montgomery Islands at the centre of the main
reef (Fig.l) consist of Quaternary sediments comprising
mud banks and sand sheets (Brooke 1995). They are a
product of marine sedimentary processes since
inundation.

The five rocky High Cliffy Islands comprise
Paleoproterozoic rocks mapped (Map Sheet SD5I-16 &15)
as possibly Pentecost Sandstone, one of the uppermost
members of the Kimberley Group. However,
archaeologist Sue O'Connor (1987) noted the presence at
Ngalanguru Island of both "quartz sandstone and
limestone". Brooke (1995) noted that these islands have
"cliffed and etched limestone shorelines formed in flat
bedded massive stromatolitic limestone and siltstone .
During a WAMSI visit to Montgomery Reef (September
2009) landings were made on three of the five High Cliffy
Islands and the following observations were made.

On top of all the High Cliffy Islands there is a cap of
blocky, bedded, strongly jointed, fine-grained quartz
sandstone or siltstone, that is the basis for mapping the
islands as Pentecost Sandstone (Fig. 5). The sandstones
overlie beds of a massive, silicified, laminated, richly
stromatolitic dolomite that outcrops along the island
shores and is the rock referred to by O'Connor and
Brooke as limestone. This rock is very hard and very
heavy and breaks into sharp edges. It was used for tool¬
making by the pre-contact inhabitants. It appears to be
an unnamed formation and is not mentioned in the
geological notes accompanying the map sheet. Some
layers of the stromatolite domes exposed at the surface
have a hard covering of secondary accretion that is stark
white and, from a distance, makes the rocks look as if
they are covered with cormorant droppings (Figs 6, 7).
On the shore, below high tide mark, the dolomite is grey
and etched (i.c. eroded into multi-faceted, sharp-edged
ridges and turrets). On the eastern shores of the islands
the top of the dolomite is up to 14 m above low tide level.
In places it is horizontally laminated and gently folded
and there is a 3—4° dip to the northwest so that, while it
forms supralittoral cliffs along the eastern shores, on
most islands it disappears into the intertidal zone along
the western or northwestern shores.

Wuljarli and Egret Islands

These islands are located on the upper level of the reef
flat at the south eastern side of the main Montgomery
Reef platform, a step up from the lower level close to the
reef front (Figs 3, 4). Like the High Cliffy Islands,
Wuljarli and Egret Islands are mapped as Pentecost
Sandstone. Landings were made on all three islands and
the rocks of them all were found to be blocky fine¬
grained quartzite like the upper rocks of the I ligh Cliffy
Islands. On the southwestern shore of Wuljarlu Island
there is also a southwest-dipping conglomerate
comprising rounded river stones in a ferrous matrix.
There was no dolomite on any of these islands.

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Journal of the Royal Society of Western Australia, 94(2), June 2011

Figure 3. The Egret Islands on the edge of the high platform on the south-eastern margin of Montgomery Reef. Note the double
containment banks of the high platform margin above the lower platform, with sand fans along their downside margin, and the
crescent-shaped pools formed by coalescing ridges of rhodoliths and coral rubble in the lagoon. (Data courtesy of WAMSI and
Airborne Research Australia, and images processed by Curtin University.)

Figure 4. Wulajarlu Isand on the rim of the upper lagoon of Montgomery Reef where it impinges on the reef-front without a lower reef
flat. This island is built of Paleoproterozoic quartz sand stone and conglomerates and there is no dolomite. (Data courtesy of WAMSI
and Airborne Research Australia, and image processed by Curtin University.)

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Wilson & Blake: Biogeomorphology of Montgomery Reef, Kimberley

Figure 5. On the top of Wulajarlu Island, looking north at high tide. Blocky quartz sandstone in the foreground, the Egret Islands on
the skyline. (Photo Barry Wilson).

Montgomery Reef and the High Cliffy Reef

While the rocks of the eastern islands are
Paleoproterozoic and probably of the Kimberley Group,
there is no published account of the rock of the reef
platforms, f.e. beneath the Holocene sediment and
crustose algal veneer. In September 2009, a WAMSI party
landed on the eastern margin of the main Montgomery
Reef opposite High Cliffy Reef during a period of neap
low tide. Standing on the mid-littoral platform behind
the reef crest, about two hundred metres from the reef
edge, there is a group of massive, silicified dolomite
rocks, the tops of which are emergent at high tide (Fig.
9). Stromatolite structures were not observed in these
rocks but the lithology of the rock is otherwise quite
similar to the stromatolitic dolomite of the High Cliffy
Islands. One flat-topped stack about 10 m in diameter,
stands 4 m high above the lower reef flat surface, its
upper part demarcated from the etched lower part by a
conspicuous bedding plane with a slight dip west. These
rocks were thought to be in situ erosional relics of the
rock platform. If that is the case, the lower, mid-littoral
rock platform on this part of the main Montgomery Reef
has a base of massive dolomite that is older than the
Pentecost Sandstone which, on the High Cliffy Islands,
overlies it. However, the thickness of the Holocene
sedimentary veneer on the reef surface around the
emergent stacks is unknown.

Intertidal habitats of the Montgomery Reef complex

The following notes provide a provisional account of
the habitats of Montgomery Reef. They are derived

mainly from reports by Brooke (1995, 1997),
interpretation of high definition imagery produced by
Curtin University for WAMSI, field surveys by W.A.
Museum and AIMS, and the recent field observations by
the authors.

High Cliffy Reef

Brooke (1997) noted some shoreline features around
Ngalanguru Island. Wells et al. (1995) referred to "gentle
terracing" of the reef platform and noted the presence of
a large lagoon. Aerial photographs (Fig. 2) show a wide
reef flat and lagoon on the western side of the High Cliffy
Islands. The western reef edge, bordering the High Cliffy
channel, appears to be poorly defined and observations
by the WAMSI party during a period of neap tides
indicated that this reef platform is lower than that of
Montgomery Reef on the opposite side of the channel.
The eastern side of High Cliffy Reef has a steep fore-reef
slope but the reef edge was not observed.

The Breakwater

The Breakwater is a long finger of reef jutting north
from the western end of the main Montgomery Reef (Fig.
1). It appears to be a distinct structure attached to the
main reef. Brooke (1997) provided a description of a site
on the western side, noting that the lower-littoral ramp is
characterized by extensive algal turf and pavement with
two to three low terraces around 20 cm high. Reef-front
coral growth did not feature in his description except for
Porites sp. rimming shallow sandy pools. The mid-littoral
reef flat is composed of a network of pools separated by

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Journal of the Royal Society of Western Australia, 94(2), June 2011

Figure 6. Top of the stromatilite dolomite forming a supra-littoral bench on a small islet south of Ngalanguru Island, High Cliffy
Group. Note the circular tops of large stromatolites in the foreground and the stark white crust over much of the rock surface. This
surface appears to be the top of the dolomite sequence exposed by removal of the sandstone above it. (Photo Barry Wilson.)

Hi^h CHffy'Group?(I’hoBarry"VV^ison) U ' 3 “ ^ °" n ° r,hem Side ° f 3 sma “ islet south of Ngalanguru Islanc

112

Wilson & Blake: Biogeomorphology of Montgomery Reef, Kimberley

uooer lagoon

upper rhodolith bank

lower lagoon

dolomite stacks

lower rhodolith bank

reef-front ramp

channel

Figure 8. The south-eastern margin of Montgomery Reef, opposite High Cliffy Reef. Note the broad reef-front ramp dissected at
intervals by drainage gutters, and the reef crest with rhodolith banks impounding shallow pools on the midlittoral reef flat with pools.
Running diagonally across the top left comer is the higher complex of rhodolith banks that impound the high lagoon. A major
drainage channel entering from the left originates in the high lagoon. (Data courtesy of WAMSI and Airborne Research Australia, and
image processed by Curtin University.)

narrow strips of algal pavement. These clear water pools
also contain abundant coarse sand and calcareous mud.

Main Montgomery Reef
a) The subtidal fore-reef

The subtidal fore-reef zone is difficult to observe. The
following notes derive from drop-camera and ROV
observations made by AIMS (Andrew Heyward pers.
com.). Around most of the reefs periphery the fore-reef is
a vertical, stepped or steeply sloping wall to a depth of
about 10 m and thence a slope to the surrounding seabed
at around 20 m. The wall appears to bear little epifaunal
or epiphytic growth. There are sand sheets in the
sublittoral zone at some localities, often with pronounced
"dune" formation, probably a result of the intense tidal
currents [up to 2 m/sec associated with an 11m tidal
range during Spring Tides. S.Blake pers. obs] recorded
around the margins of Montgomery Reef. A fore-reef
spur and groove system is not present. Nor is there a rich
community of large Porites sp. and foliaceous corals like
those known to occur in this zone of those few Kimberley
fringing reefs that have been studied. However, there are
many shallow subtidal ledges and patch reefs that bear
moderately diverse coral communities. The seabed
beyond the fore-reef is a rocky pavement with little
sediment and bears well developed though patchy filter¬

feeding communities of mainly sponges, sea-whips, sea-
fans and soft corals.

b) Lower-littoral - reef-front ramp

The lower-littoral reef-front of Montgomery is a high-
energy environment, not from wave action but from
intense off-reef tidal flow. It is an extreme habitat where
calcareous algae and low turf algae flourish but coral
colonies are small and sparse. The huge volume of water
impounded above the two impoundments banks and the
many lesser terraces on the reef flat, results in
spectacular, high velocity cascades over the terraces
where the reef-front is high and steep and, in spring tide
periods, over the reef front (Fig. 10). At such times, the
larger drainage channels across the lower reef platform
act like mountain rivers with extremely turbulent flow.

The reef-front ramp around Montgomery Reef is
clearly evident in aerial photographs as a prominent zone
varying in width from 50 to 100 m. It is generally high
and steep, sloping at around 5-10° or more, sometimes
convex, and has a distinct reef edge. At Spring Low Tide
the reef edge may be several meters above water level
(Figure 10). The ramp pavement is covered with a
calcareous algal crust and a low turf. In most places the
ramp is distinctly terraced by ridges of crustose algae a
few cm high that impound networks of shallow pools,
rather like a miniature Javanese hillside with rice

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Journal of the Royal Society of Western Australia, 94(2), June 2011

Figure 9. A massive dolomite stack on the mid-littoral reef flat near the south-eastern margin of Montgomery Reef (15°55.22rS;
124°18.795’E). Etched rocks on the lower part (i.e. below high tide level). Also visible as a cluster of small dots on Figure 8. (Photo Steve
Blake, WAMSI.)

Figure 10. The reef edge at low tide; major channel, south west of Wulajarlu Island, Montgomery Reef. (Image courtesy Tim Willings,
Pearl Sea Coastal Cruises)

terraces. The depth of the algal crust is unknown but is
probably superficial.

Corals in the reef-front zone may be common, sparse
or lacking. When present, corals are small colonies with
flattened morphologies, mostly growing around the
edges of the shallow pools. There is no zone of prolific
coral growth along the reef-front like that of fringing
reefs in the Bonaparte Archipelago and no evidence of a

Holocene coralline limestone wedge and outward reef
growth at the reef-front.

c) Mid-littoral — reef crest

A boulder zone is lacking but commonly there is a
reef crest equivalent comprising an elevated bank of
rhodoliths up to 100 m wide fronted by a narrow fan of
coarse sand (Figs 11, 12, 13). Unlike the boulders of coral

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Wilson & Blake: Biogeomorphology of Montgomery Reef, Kimberley

reef crests that are derived from the fore-reef and
deposited by wave action, the rhodoliths appear to be
derived from the sandy pools of the mid-littoral lagoon
behind the reef crest and deposited there by ebbing tidal
flow and wind-driven waves. The rhodolith banks are
mobile, moved back and forth over the reef crest zone by
tidal flow. They are generally very long and wind their
way around the reef crest zone, often dividing into
multiple bands that coalesce. The rhodolith banks are not
terraces in the strict sense but they impound water
behind and between them. The rhodoliths are irregular
in form. Some have a core of coral fragments but many
are more or less globular, ranging in diameter from 5 cm
to 12 cm and are of entirely algal construction (Fig. 13).

d) Mid-littoral reef flat

The reef flat of Montgomery Reef, behind the reef
crest, is unusually high in the intertidal zone and, at least
on the eastern side, is formed at two distinct levels that
are separated by an upper rhodolith impoundment bank
(Figs 8, 11).

(i) Lower reef flat

The lower mid-littoral reef flat behind the reef crest
varies in its nature, sometimes being a typical reef flat
with an exposed pavement, crustose surface and low
algal turf, and sometimes comprising a mosaic of
shallow, knee-deep to waist-deep (at low tide) pools
separated by secondary rhodolith ridges or crustose
coralline algal ridges, the tops of which may be exposed.
Aerial photographs show that the ridges are crescent¬
shaped and coalescent (Figs 3, 8) with steep outer
margins and sloping inner margins, indicating that they
are formed by the force of the ebb tide. In areas where
pools dominate, this habitat might be called a lower mid¬

littoral lagoon rather than a reef flat, but neither term is
strictly apt.

The pools are lenticular, with sand, rhodolith and
rubble beds and contain moderately diverse coral
communities, leafy brown algae and some seagrass
(Thalassia). Similar configurations of rhodolith ridges and
pools were observed at Turtle Reef in the nearby Talbot
Bay on Yampi Peninsula (Wilson et al, this volume. Fig.
9). The abundant rhodoliths apparently grow in the pools
where they are rolled by the tide to form the networks of
ridges and eventually up onto the reef crest where they
build the banks that rim and impound the pools and
lagoon.

(ii) Upper reef flat (lagoon)

The upper reef flat is actually a shallow lagoon
occupying most of the area of Montgomery Reef although
there are some areas of rock pavement exposed at low
tide. Little information is available at this time on the
extent of coral and rhodolith growth in this zone. Aerial
photographs show that the high lagoon includes areas
where shallow, lenticular pools dominate, separated by
coalescing ridges of rhodoliths and rubble, like those of
the lower reef flat/lagoon so that vigorous rhodolith
growth may be inferred.

The upper and lower platforms are demarcated by the
upper complex of rhodolith banks, with a sand fan in
front of it (Fig. 3). This upper terrace is virtually
continuous and almost encircles Montgomery Reef and is
clearly evident in the satellite images and the aerial
photographs (Figures 1, 3, 11). Its height is not known.
This conspicuous feature may be related to the
underlying geological structure or a historical
constructional feature relating to a Holocene (eustatic)

^ Wuiaiarlu Island

dolomite slacks

X

Eeret Island ^

lower laeoon

*\ipper rhodolith bank

UDDer laeoon

lower laeoon

Figure 11: Rhodolith banks, lagoons and reef-front ramps along the S.E margin of Montgomery Reef. In the right fore-ground there are
several rhodolith banks forming steps down from the upper lagoon to a mid-littoral reef flat. In the centre a tongue of the upper lagoon
stretches to the reef edge and there is no mid-littoral reef flat. Wulajarlu Island top left; Egret Island top right margin. (Photo Steve
Blake, WAMSI)

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Journal of the Royal Society of Western Australia, 94(2), June 2011

Figure 12. Rhodolith bank on the reef crest; S.E. Montgomery Reef (15°55.33rE; 124°18.85rE; elevation c. 4 m). Mid-littoral reef flat on
the left; High Cliffy Reef and islands in the background. (Photo Barry Wilson.)

Figure 13. Rhodoliths on the reef crest (see Fig. 12). Some of these have a coral fragment core but the majority are entirely of algal
construction. (Photo Barry Wilson.)

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Wilson & Blake: Biogeomorphology of Montgomery Reef, Kimberley

high sea level. However, it is more likely to be a
contemporary constructional, biogenic feature relating to
tidal level.

The area occupied by the upper reef flat lagoon is
enormous (c. 350 km 2 ). It forms a vast light trap at low
tide, with shallow conditions where autotrophic
organisms flourish without exposure to the air or
significant wave action (except during cyclonic storms).
Primary productivity in these intertidal lagoons has not
yet been measured but must be prodigious.

Summary and discussion

Reef growth

While there is a diverse coral fauna on Montgomery
Reef, there is no prolific coral growth on the reef-front
edge or the fore-reef wall and no evidence of the
development of a reef-front biogenic limestone wedge. In
other words, there is little reef-building activity in the
reef-front zone and no significant lateral reef growth.
However, there is vigorous biological growth of both
rhodoliths and scleractinian corals in the pools and
lagoons of the intertidal platform behind the rhodolith
impoundment banks. Biogenic rubble and sand produced
by these processes is filling in the pools and lagoons and
raising the level of the platform.

The rhodolith banks cover large areas of the reef crest,
pools and lagoons of the reef flat and clearly play a
significant reef-building role. It would be interesting to
obtain estimates of the rates of growth of the rhodoliths
and corals but the impression gained from visual
observations is that the rhodoliths are the primary reef¬
building organism on the Montgomery Reef platform.

The presence and configuration of these ridges and
banks of rhodoliths and algal terraces, and the pools and
lagoons they create, are functions of the macro-tidal
regime and an unusual biogeomorphic feature. Kuenen
(1933) discussed the importance of "lithothamnium" in
reef-building on coral reefs and mentioned a report of
the Siboga Expedition (Weber 1902) of 'Tithothamnium
in loose nodules covering bare reefs ... in several parts of
the East Indies". This latter study has not been seen but
is the only reference known to these authors to large scale
intertidal rhodolith banks and significant reef-building
by these organisms.

It is suggested that calcareous algae may be the
predominant reef-building organisms on Montgomery
Reef, probably more significant that the corals, affecting
the process in two ways:

• by their own carbonate production in the shallow
pools and lagoons of the reef platform, and

• by their construction of the rhodolith banks and
terraces around the perimeter of the reef that create
the lagoonal habitat where the bulk of the
carbonate production takes place (by rhodoliths
and corals).

T he vast area (c. 350 km 2 ) of shallow, sheltered, sunlit,
lagoon and pools of the Montgomery Reef platform
represents a very significant extent of Benthic Primary
Production Habitat. High primary production in the
impounded lagoons may be the explanation of the

abundance of herbivorous macrofauna, notably green
turtles and dugong, for which this area is renowned. If
this is true, the rhodolith banks may be responsible for it.

Age of the islands

In the mainland area adjacent to Montgomery Reef,
the second youngest unit in the Kimberley Group series
is the Pentecost Sandstone and it is underlain by the
Elgee Siltstone. Both these units are older than 1790±4
Ma, based on zircon dating of the intrusive Hart Dolerite
(Schmidt and Williams 2008). Assuming that the quartz
sandstone of Wuljarli and the Egret Islands and atop the
High Cliffy Islands is correctly mapped as Pentecost
Sandstone, the rocks of those islands are Paleoproterozoic
and around 1.8 billion years old.

Identity and age of the High Cliffy stromatolites

Commonly, along the eastern shores of the High Cliffy
Islands, the top of the dolomite bed is exposed as a
supra tidal bench around 12-14 m above low tide level. In
those situations the stromatolites are exposed on the
surface as well preserved dome-shaped bosses, up to 1 m
in diameter (Fig. 6). On worn, flat surfaces, they appear
as finely laminated concentric rings. Where the
stromatolites are exposed in section on cliff faces, they
appear as cone-shaped structures with irregular,
drooping layers at the top (Fig. 7). At one location a
cluster of massive stromatolites was observed comprising
rounded turrets standing up to 30 cm high, and
occupying an area of around 20 m (Fig. 7).

The High Cliffy stromatolites were first reported by
Mr Kevin Coates who took specimens to Dr Kathleen
Grey of the Western Australian Geological Survey in the
late 1980s. Dr Grey identified them as a previously
unknown form of conical stromatolites belonging to the
Group (morpho-genus) Conophyton. She has confirmed
that specimens collected by the WAMSl party in
September 2009 are of the same kind (Grey pers. com.).
These stromatolites are unlike any Form (morpho-
species) known from the Kimberley area. Conophyton
usually indicates quiet water conditions, below the wave
base and the High Cliffy examples represent a biohermic
construction in the Kimberley Basin very early in the
history of life on this planet.

At the time of this report, the High Cliffy stromatolites
remain undescribed and their stratigraphic interpretation
is uncertain. The Group Conophyton is common
throughout the Proterozoic. It ranges into the Cambrian
and there are modern analogues, so the age of the
succession cannot be determined until the taxon can be
identified to Form level although stratigraphically a
Paleoproterozoic age is indicated.

In regard to the age and stratigraphic relationships of
the stromatolitic dolomite, and noting that on the High
Cliffy Islands it underlies the quartz sandstone mapped
as Pentecost Sandstone, Dr Grey (pers. com.) suggested
three possible interpretations:

• It is a hitherto unknown carbonate unit within the
Pentecost Sandstone.

• It is a previously unrecognized facies within the
older Elgee Siltstone.

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Journal of the Royal Society of Western Australia, 94(2), June 2011

• The stromatolite unit and overlying sandstone of
the High Cliffy Islands are not part of the
Kimberley Group, but are either a younger or
older sedimentary package that has not yet been
recognized elsewhere in the Kimberley.

Age of the rocks of Montgomery Reef platform

If the dolomite stacks of the lower platform at the
eastern end of the main Montgomery Reef are correctly
correlated with the stromatolitic dolomite of the High
Cliffy Islands, it may be assumed that the rock of the
lower platform in that area is also of Paleoproterozoic
age. The age of the rocks of the upper platform remains
conjectural. Because they are higher than the dolomite of
the lower platform it is possible that they are Pentecost
Sandstone, like the Wuljarli and Egret Islands.

The age of the contemporary biogenic limestone that
veneers the Proterozoic rocks of the reef platform surface
is certainly Holocene. It is possible that there is
Pleistocene coralline limestone beneath the Holocene
veneer but no surface exposure of it was seen on
Montgomery Reef (or anywhere else in the Kimberley
north of Cape Levcque). There is evidence of ongoing
subsidence of the continental margin in the Kimberley
(Gregory 1913; Teichert and Fairbridge 1948; Fairbridge
1953; Carrigy and Fairbridge 1964; Jongsma 1970;
Sandiford 2007) in which case Pleistocene reef limestone,
if it exists, is likely to be at some depth below the
Montgomery Reef platform surface.

Age and formation of the reef platform

While the base rocks of the Montgomery Reef
structure arc probably of early Proterozoic age, the means
and the time at which the flat reef platform was created
are conjectural. There are several possibilities.

1. A wave-cut rock platform

Edwards (1958) described wave-cut rock platforms
around the shores of islands in the Buccaneer
Archipelago and the mainland of the Yampi Peninsula.
He noted that such rock platforms arc best developed
where the shore rocks are quartz-feldspar porphyry or
schists that weather easily and poorly developed where
the rocks are quartzites. The rock exposures of the islands
of Montgomery Reef are all hard quartzites or silicified
dolomite and there are no wave-cut rock platforms
around their shores. It is improbable that the intertidal
platform of Montgomery Reef could be an erosional
surface created by wave action and chemical erosion
during the Holocene.

2. A Quaternary coral reef platform built on and around a
Proterozoic rock core.

Such a process would require extremely rapid reef
growth to create a platform reef as large as Montgomery.
It could be possible if there were a pre-existing Late
Pleistocene reef (as at Ningaloo Reef) upon which
Holocene reef growth occurred. However, the evidence
of subsidence in the region suggests that this is unlikely.
Also, if there were rapid contemporary reef growth,
vigorous reef-front and fore-reef coral communities
would be expected. This does not appear to be the case.
Those habitats are poorly populated by corals and there
is no evidence of lateral reef growth. Upward growth on

the reef platform would level and raise the surface but
would not create a reef platform on the scale of
Montgomery Reef without there being lateral growth as
well.

3. A pre-existing flat terrestrial erosional surface

Prior to the post-Last Glacial Maxima transgression,
the Montgomery structure would have stood as a
mountain on a plain many kilometres from the coast. The
rocks of which it is built are probably upper units of the
Kimberley Group. In this regard, Montgomery Reef is
like the fringing reefs of the Kimberley Bioregion where
Holocene biogenic growth appears to be built directly on
Proterozoic rocks of the Kimberley Group, except that at
Montgomery there is no evidence of a Holocene biogenic
limestone wedge at the reef-front.

We suggest that, prior to the Last Glacial Maxima, the
Montgomery structure was a flat-topped terrestrial mesa.
There are analogues of such structures, of that age and of
similar height and area, further inland in the Kimberley
Basin in a similar position in relation to the boundary
between the basin and the King Leopold Orogen ( e.g .
Mount House, Mount Clifton). The primary geomorphic
features of Montgomery Reef today are the same as those
of the inland mesas - flat top, vertical walls, peripheral
canyon-like incisions. By this interpretation, the main
geomorphic features of Montgomery Reef, including its
flat top, have been inherited from its long history of
terrestrial erosion.

Conversion of the terrestrial Montgomery mesa to a
marine platform reef would have occurred in the
Holocene with the advent of the post-Last Glacial
Maxima transgression. Mean Sea Level rose to just above
the flat top of what was previously the mesa, placing it
within the intertidal zone of a macro-tidal shore, open to
colonization by intertidal marine organisms. Geomorphic
features of the reef surface today (central mud islands,
sand sheets, lagoons and rhodolith banks) are results of
contemporary marine coral and algal reef-building
processes superimposed on pre-existing terrestrial
features, perhaps further leveling the intertidal platform
surface by means of biogenic growth and sedimentation.

Conclusions

Montgomery Reef is not a coral platform reef in the
strict sense. It is a coral reef in the ecological sense of
having diverse coral growth in the lagoons and pools of
the intertidal platform but its geomorphic form is
atypical. There is no evidence of lateral reef growth and
it does not have a biogenic limestone framework. Rather,
the evidence suggests that it is an inundated terrestrial
structure, built of Paleoproterozoic metasedimentary
rocks, with a Holocene veneer of biogenic limestone and
sediment on its intertidal platform surface. The thickness
of the Holocene veneer is unknown but is probably not
great. A drilling program on the reef flat would be
required to confirm this interpretation.

The reef has inherited its primary geomorphic
features, including its level platform, from its terrestrial
erosional history. Contemporary geomorphic processes
on the reef platform are constructional and involve
production and distribution of modern biogenic

118

Wilson & Blake: Biogeomorphology of Montgomery Reef, Kimberley

sediments. Extreme macro-tidal conditions and wind-
driven waves are the dominant forces involved and the
formation of vast mobile banks of rhodoliths that
impound shallow lagoons is the key factor that creates
the highly unusual intertidal habitats of the reef
platform.

Acknowledgements: Information and comments provided by Andrew
Heyward (AIMS) and Clay Bryce (W.A. Museum) are gratefully
acknowledged. TO Isobel Peters and guide, Roy Wiggan, contributed vital
local knowledge and guidance. From data produced by the School of the
Environment , Airborne Research Australia, Flinders University, Adelaide,
Curtin University produced extremely detailed aerial photographs and
hyperspectral images of the reef, some of which are reproduced here. The
skills and good company of skipper Andy Passmore and crew of the
Fisheries vessel P.V. Walcott are also greatly appreciated. Field work was
supported by the Western Australian Marine Science Institution.

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